In DE 44 45 829 A1 (Leybold A G) a counter-flow sniffing leak detector is described which comprises a high-vacuum pump stage at the end of a sniffing hose. An approximately 4 m long sniffing hose configured as a capillary line and having an inner diameter of approximately 0.4 mm is used. The sniffing hose exerts the required throttling effect for maintaining the vacuum generated at its outlet end.
DE OS 24 41 124 describes a leak detecting apparatus comprising a sniffing hose, where the hose has a relatively large diameter. Immediately in front of the test gas sensor, which is a mass spectrometer, a throttle is provided. Since a throttle is arranged immediately in front of the test gas sensor, the gas to be detected can be more rapidly delivered from the inlet of the sniffing probe to a place immediately in front of the throttle, i.e. to the test gas sensor, with the aid of the vacuum pump. Thus the response time, which depends on the length of the hose line, is reduced.
The applicant offers, inter alia, a device family comprising the ECOTEC and PROTEC devices which are leak detectors having a sniffing probe. The two devices differ from each other, inter alia, by the cross section of the capillary tube which connects the sniffing probe with the main device. The flow in the capillary tube is caused by the pressure difference between the inlet of the sniffing probe and the outlet end of the capillary tube at the main device, where a rough vacuum (p<250 mbar) is produced. In the main device the test gas proportion is permanently determined by the test gas sensor. From the measured test gas concentration and the gas flow, the leak rate of the leakages is determined.
Irrespective of the response time of the test gas sensor in the main device, the period between the gas entry into the sniffing probe and the signal indication is determined by the time it takes for the gas to pass through the capillary tube. This time is the so-called dead time. The longer the capillary tube is, the longer is the dead time of the system. At a predetermined capillary tube length, the dead time can be influenced by changing the flow velocity. The flow velocity, in turn, can be changed by changing the tube cross section and the pressure difference between the inlet and the outlet end of the capillary tube.
It is desirable to select as large a flow as possible in the sniffing probe such that the dead time is minimized and a sufficient amount of air is taken into the sniffing probe even from a larger distance. The test gas concentration c in the gas flow of the air taken in is shown by the following equation:
  c  =      L          q      pV      where c is the concentration of the leak gas, L is the leak rate, and qpV is the gas flow through the capillary tube.
The smallest traceable leak rate corresponds to the smallest detectable test gas concentration. Thus the smallest traceable leak rate is impaired when the gas flow in the sniffing probe increases. Accordingly, the selected flow is always a tradeoff between the boundary conditions of minimum dead time and smallest traceable leak rate.